The US had staged an unsuccessful operation this summer to rescue American hostages held in Syria by the militant group ISIL, the Pentagon has said.

The disclosure came a day after the Islamic State of Iraq and the Levant released a disturbing video showing a member of the extremist group beheading American journalist James Foley, who was abducted abducted in Syria in November 2012.

"This operation involved air and ground components and was focused on a particular captor network within ISIL. Unfortunately, the mission was not successful because the hostages were not present at the targeted location," Pentagon Press Secretary John Kirby said.

Earlier this summer, President Barack Obama had authorized an operation to attempt the rescue of Americans who were kidnapped by the ISIL and held in Syria, said Lisa Monaco, Assistant to President for Homeland Security and Counter -terrorism.

Obama authorized action at this time because it was the national security team's assessment that these hostages were in danger with each passing day in ISIL custody, she said.

Monaco said the US Government had what it believed was sufficient intelligence, and when the opportunity presented itself, the President authorized the Department of Defense to move aggressively to rescue the hostages.

"Unfortunately, that mission was ultimately not successful because the hostages were not present," she said, adding "given the need to protect our military's operational capabilities, we will not be able to reveal the details of this operation."

Meanwhile Kirby said that the US was committed to the safety and well-being of its citizens, particularly those suffering in captivity.

"In this case, we put the best of the United States military in harms' way to try and bring our citizens home," he said.

"The US government uses the full breadth of our military, intelligence and diplomatic capabilities to bring people home whenever we can. America will not tolerate the abduction of our people, and will work tirelessly to secure the safety of our citizens and to hold their captors accountable," the Pentagon spokesperson said.

Meanwhile, Caitlin Hayden, Spokesperson of the National Security Council, said: "We never intended to disclose this operation. An overriding concern for the safety of the hostages and for operational security made it imperative that we preserve as much secrecy as possible."

"We only went public today when it was clear that a number of media outlets were preparing to report on the operation and that we would have no choice but to acknowledge it," Hayden said.

"In this case, we put the best of the US military in harms' way to try and bring our citizens home," the Pentagon spokesperson said.

"The US government uses the full breadth of our military, intelligence and diplomatic capabilities to bring people home whenever we can. America will not tolerate the abduction of our people, and will work tirelessly to secure the safety of our citizens and to hold their captors accountable," he said.

The White House and the Pentagon decided to go public with the information as it came to know that many of the media outlets had come to know about this secretive operation.

"We never intended to disclose this operation. An overriding concern for the safety of the hostages and for operational security made it imperative that we preserve as much secrecy as possible," said Caitlin Hayden, Spokesperson of the National Security Council, White House.

"We only went public today when it was clear a number of media outlets were preparing to report on the operation and that we would have no choice but to acknowledge it," Hayden said.

According to unnamed official who spoke to a select group of reporters on condition of anonymity there were a "good number" of militant casualties at the site, but that one US service member received a "minor injury when one aircraft did take some fire."

Foley's beheading marks the first time the Islamic State has killed a US citizen since the Syrian conflict broke out in March 2011. Opposition activists say the war has killed more than 170,000 people.

The militant group has threatened to kill another reporter unless the US halts its air strikes in Iraq.

President Obama authorised "targeted air strikes" against Islamic State militants early this month to protect American military personnel and prevent potential "genocide" of thousands of displaced religious minorities trapped on a mountaintop in northwest Iraq.